Astron. Astrophys. Suppl. Ser. 131, 197-208

F, G and K stars in the ROSAT all-sky survey^,

I. Photometry

A.D.F. Metanomski^1,2 - L. Pasquini² - J. Krautter¹ - G. Cutispoto³ - T.A. Fleming^4,5

Send offprint request: A. Metanomski, e-mail: ametanom@mail.lsw.uni-heidelberg.de

1 - Landessternwarte Heidelberg, Königstuhl, D-69117 Heidelberg, Germany
2 - European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
3 - Osservatorio Astrofisico di Catania, v.le A. Doria 6, I-95125 Catania, Italy
4 - Max-Planck-Institut für Extraterrestrische Physik, Karl-Schwarzschild-Str. 2, D-85740 Garching, Germany
5 - Steward Osbervatory, University of Arizona, Tucson, AZ 85721, U.S.A.

Received May 20; accepted December 9, 1997

Abstract:

We present accurate $BV(RI)_{\rm c}$ photometry for a sample of F, G and K stars detected in selected areas of the ROSAT all-sky survey (RASS). We have used the photometry, in addition to low-resolution spectroscopy, to estimate spectral classifications, distances and X-ray luminosities. The $\log (L_{\rm X} / L_V)$ in the sample lies below -2. Although the sample contains also nearby, inactive stars, it is dominated by active objects. The median X-ray luminosity in our sample is $< L_{\rm X} \gt = 29.88$ and the mean value of the hardness ratios $<{\rm HR}1\gt = 0.13 \pm 0.35$.

We compare the derived X-ray luminosity function with similar functions obtained from the serendipitous samples of the Einstein Observatory medium sensitivity survey (EMSS) and EXOSAT.

Our sample is completely consistent with the EMSS sample of solar type stars, indicating that both our sources and the EMSS sources are representative of the high galactic latitude X-ray stellar population. We do not find extremely active stars ($\log (L_{\rm X}) \ge 32$), as are found in the EMSS sample, and we argue that these objects are rare.

Key words: stars: late-type -- X-ray: stars -- surveys